"Вони їдуть на автобусі."

Translation: They are going by bus.

[Tanyabon]

They ride a bus

[MikheevSasha]

They are riding on a bus should be also accepted

[nairamilene]

For transportation in Ukrainian language the correct is to use instrumental case Вони їдуть автобусом ( автобусом means by car) ( на автобусі means "in the car")

[va-diim]

Автобус means "bus," not "car."

[shrolhei]

Why is THE bus not accepted

[sagitta145]

Accepted now for sure! (don't know since when)

[lttl_grimmy]

Why is they are going on a bus not accepted?

[sagitta145]

It is. You probably made a typo or some other mistake.

For example, I see a report submitted by someone asking to accept "The are going on a bus". Could be you? :)